Growth and differentiation factor-8 (GDF-8), also known as myostatin, is a member of the transforming growth factor-beta (TGF-β) superfamily of structurally related growth factors, all of which possess important physiological growth-regulatory and morphogenetic properties (Kingsley et al. (1994) Genes Dev., 8: 133–46; Hoodless et al. (1998) Curr. Topics Microbiol. Immunol., 228: 235–72). GDF-8 is a negative regulator of skeletal muscle mass, and there is considerable interest in identifying factors which regulate its biological activity. For example, GDF-8 is highly expressed in the developing and adult skeletal muscle. The GDF-8 null mutation in transgenic mice is characterized by a marked hypertrophy and hyperplasia of the skeletal muscle (McPherron et al. (1997) Nature, 387: 83–90). Similar increases in skeletal muscle mass are evident in naturally occurring mutations of GDF-8 in cattle (Ashmore et al. (1974) Growth, 38: 501–507; Swatland and Kieffer (1994) J. Anim. Sci., 38: 752–757; McPherron and Lee (1997) Proc. Nat. Acad. Sci. U.S.A., 94: 12457–12461; and Kambadur et al. (1997) Genome Res., 7: 910–915). Recent studies have also shown that muscle wasting associated with HIV-infection in humans is accompanied by increases in GDF-8 protein expression (Gonzalez-Cadavid et al. (1998) Proc. Natl. Acad. Sci. U.S.A., 95: 14938–43). In addition, GDF-8 can modulate the production of muscle-specific enzymes (e.g., creatine kinase) and modulate myoblast cell proliferation (WO 00/43781).
A number of human and animal disorders are associated with loss of or functionally impaired muscle tissue. To date, very few reliable or effective therapies exist for these disorders. However, the terrible symptoms associated with these disorders may be substantially reduced by employing therapies that increase the amount of muscle tissue in patients suffering from the disorders. While not curing the conditions, such therapies would significantly improve the quality of life for these patients and could ameliorate some of the effects of these diseases. Thus, there is a need in the art to identify new therapies that may contribute to an overall increase in muscle tissue in patients suffering from these disorders.
In addition to its growth-regulatory and morphogenetic properties in skeletal muscle, GDF-8 may also be involved in a number of other physiological processes (e.g., glucose homeostasis), as well as abnormal conditions, such as in the development of type 2 diabetes and adipose tissue disorders, such as obesity. For example, GDF-8 modulates preadipocyte differentiation to adipocytes (Kim et al. (2001) B.B.R.C. 281: 902–906). Thus, modulation of GDF-8 may be useful for treating these diseases, as well.
The GDF-8 protein is synthesized as a precursor protein consisting of an amino-terminal propeptide and a carboxy-terminal mature domain (McPherron and Lee, (1997) Proc. Nat. Acad. Sci. U.S.A., 94: 12457–12461). Before cleavage, the precursor GDF-8 protein forms a homodimer. The amino-terminal propeptide is then cleaved from the mature domain. The cleaved propeptide may remain noncovalently bound to the mature domain dimer, inactivating its biological activity (Miyazono et al. (1988) J. Biol. Chem., 263: 6407–6415; Wakefield et al. (1988) J. Biol. Chem., 263: 7646–7654; and Brown et al. (1990) Growth Factors, 3: 35–43). It is believed that two GDF-8 propeptides bind to the GDF-8 mature dimer (Thies et al. (2001) Growth Factors, 18: 251–259). Due to this inactivating property, the propeptide is known as the “latency-associated peptide” (LAP), and the complex of mature domain and propeptide is commonly referred to as the “small latent complex” (Gentry and Nash (1990) Biochemistry, 29:6851–6857; Derynck et al. (1995) Nature, 316:701–705; and Massague (1990) Ann. Rev. Cell Biol., 12: 597–641). Other proteins are also known to bind to GDF-8 or structurally related proteins and inhibit their biological activity. Such inhibitory proteins include follistatin (Gamer et al. (1999) Dev. Biol., 208: 222–232). The mature domain of GDF-8 is believed to be active as a homodimer when the propeptide is removed.
Clearly, GDF-8 is involved in the regulation of many critical biological processes. Due to its key function in these processes, GDF-8 may be a desirable target for therapeutic intervention. In particular, therapeutic agents that inhibit the activity of GDF-8 may be used to treat human or animal disorders in which an increase in muscle tissue would be therapeutically beneficial.
Known proteins comprising at least one follistatin domain play roles in many biological processes, particularly in the regulation of TGF-β superfamily signaling and the regulation of extracellular matrix-mediated processes such as cell adhesion. Follistatin, follistatin related gene (FLRG, FSRP), and follistatin-related protein (FRP) have all been linked to TGF-β signaling, either through transcriptional regulation by TGF-β (Bartholin et al. (2001) Oncogene, 20: 5409–5419; Shibanuma et al. (1993) Eur. J. Biochem. 217: 13–19) or by their ability to antagonize TGF-β signaling pathways (Phillips and de Kretser (1998) Front. Neuroendocrin., 19: 287–322; Tsuchida et al. (2000) J. Biol. Chem., 275: 40788–40796; Patel et al. (1996) Dev. Biol, 178: 327–342; Amthor et al. (1996) Dev. Biol., 178: 343–362). Protein names in parentheses are alternative names.
Insulin growth factor binding protein 7 (IGFBP7, mac25), which comprise at least one follistatin domain, binds to insulin and blocks subsequent interaction with the insulin receptor. In addition, IGFBP7 has been shown to bind to activin, a TGF-β family member (Kato (2000) Mol. Med., 6:126–135).
Agrins and agrin related proteins contain upwards of nine follistatin domains and are secreted from nerve cells to promote the aggregation of acetylcholine receptors and other molecules involved in the formation of synapses. It has been suggested that the follistatin domains may serve to localize growth factors to the synapse (Patthy et al. (1993) Trends Neurosci., 16: 76–81).
Osteonectin (SPARC, BM40) and hevin (SC1, mast9, QR1) are closely related proteins that interact with extracellular matrix proteins and regulate cell growth and adhesion (Motamed (1999) Int. J. Biochem. Cell. Biol., 31: 1363–1366; Girard and Springer (1996) J. Biol. Chem., 271: 4511–4517). These proteins comprise at least one follistatin domain.
Other follistatin domain proteins have been described or uncovered from the NCBI database (National Center for Biotechnology Information, Bethesda, Md., USA), however their functions are presently unknown. These proteins include U19878 (G01639, very similar to tomoregulin-1), T46914, human GASP1 (GDF-associated serum protein 1; described herein; FIG. 7), human GASP2 (WFIKKN; Trexier et al. (2001) Proc. Natl. Acad. Sci. U.S.A., 98: 3705–3709; FIG. 9), and the proteoglycan family of testican (SPOCK) proteins (Alliel et al.(1993) Eur. J. Biochem., 214: 347–350). Amino acid and nucleotide sequences for mouse GASP1 (FIG. 6) and mouse GASP2 (FIG. 8) were also determined from the Celera database (Rockville, Md.). As described herein, the nucleotide sequence of cloned mouse GASP1 matched the predicted Celera sequence, with the exception of some base substitutions in wobble codons that did not change the predicted amino acid sequence (see FIG. 13).